This composition, generally associated with an adhesive (tackifying) resin makes it possible to produce adhesive bondings which resist elevated temperature after crosslinking of the free --NCO functional groups, usually under the effect of atmospheric moisture.
The adhesives are generally in solid form and are applied as a melt phase; they become rigid after application while providing the bonding of the surfaces to be joined. This rigidification usually results from the polymerization or polycondensation of the base components of the adhesive by postcrosslinking. For a number of years attempts have increasingly been made also to introduce adhesive compositions in solid form. In this case these are hot-melt adhesives which have, as base, a thermoplastic resin which is solid at ambient temperature, possessing adhesive properties on postcrosslinking. These adhesives are fluidized when hot, the bonding of the surfaces to be joined being ensured when the adhesive becomes rigid again on cooling. These hot-melt adhesives generally result from the combination of two base constituents: a thermoplastic resin and an adhesive (tackifying) resin, which can be used in combination with additives such as wax, stabilizer, filler, plasticizers and the like. The best-known base thermoplastic resins are polyamides, atactic polypropylene and, in particular, ethylene-vinyl acetate (EVA) copolymers. In the present state these hot-melt adhesives have good adhesive properties but have the disadvantage of exhibiting poor heat resistance, a resistance which barely exceeds 70 to 80.degree. C.